Observations and Modeling of the Mars Low‐Altitude Ionospheric Response to the 10 September 2017 X‐Class Solar Flare

Abstract

Solar extreme ultraviolet and X‐ray photons are the main sources of ionization in the Martian ionosphere and can be enhanced significantly during a solar flare. On 10 September 2017, the Mars Atmosphere and Volatile EvolutioN orbiter observed an X8.2 solar flare, the largest it has encountered to date. Here we investigate the ionospheric response before, during, and after this event with the SuperThermal Electron Transport model. We find good agreement between modeled and measured photoelectron spectra. In addition, the high photoelectron fluxes during the flare provide adequate statistics to allow us to clearly and repeatedly identify the carbon Auger peak in the ionospheric photoelectron energy spectra at Mars for the first time. By applying photochemical equilibrium, O 2+ and CO 2+ densities are obtained and compared with Mars Atmosphere and Volatile EvolutioN observations. The variations in ion densities during this event due to the solar irradiance enhancement and the neutral atmosphere expansion are discussed.

Magnetic Topology Response to the 2003 Halloween ICME Event at Mars

“…(2) When the foot point(s) of a field line is embedded in the dayside ionosphere, ionospheric photoelectrons can be observed in the field‐aligned direction(s) (e.g., Xu, Mitchell, Liemohn, et al, ; Xu, Mitchell, Luhmann, et al, ; Xu, Mitchell, et al, ). Martian ionospheric photoelectrons can be identified through typical spectral features (e.g., Coates et al, ; Frahm et al, ; Liemohn et al, ; Mitchell et al, ; Peterson et al, ; Xu, Thiemann, et al, ): a cluster of peaks from 22 to 27 eV from the 30.4 nm He‐II solar line ionizing CO 2 and O, a sharp flux drop from 60 to 70 eV (termed “the photoelectron knee”), corresponding to a sharp decrease of solar irradiance at wavelengths shorter than 17 nm, and also the large flux decrease near 500 eV as the source term is extremely small for wavelengths <∼1 nm. Similarly, photoelectrons seen in one or both field‐aligned directions indicate an open or closed field line, respectively.…”

Observations and Impacts of the 10 September 2017 Solar Events at Mars: An Overview and Synthesis of the Initial Results

“…Xu, Thiemann, et al () investigated the low‐altitude ionospheric response to the X8.2 flare by SuperThermal Electron Transport (STET) model to simulate the photoelectron spectra and ion production rates and densities for periods before the flare, during the flare peak, and during the first MAVEN in situ observations of the period after the flare peak. A good agreement was found between the modeled photoelectron spectra and observations.…”

“…In terms of compositional changes in the ionosphere, Thiemann et al (2018) reported an increase in the production of O and thus an increase in the relative abundance of O at a fixed pressure, which has implications for photochemical escape (section 6). Xu, Thiemann, et al (2018) investigated the low-altitude ionospheric response to the X8.2 flare by SuperThermal Electron Transport (STET) model to simulate the photoelectron spectra and ion production rates and densities for periods before the flare, during the flare peak, and during the first MAVEN in situ observations of the period after the flare peak. A good agreement was found between the modeled photoelectron spectra and observations.…”

The Morphology of the Topside Martian Ionosphere: Implications on Bulk Ion Flow

“…On the dayside, the Martian ionosphere contains a well-defined primary layer and a low-altitude secondary layer which are mainly produced by solar Extreme Ultraviolet (EUV)/X-ray ionization along with impact ionization by photoelectrons and their secondaries (e.g., Fox & Weber, 2012;Fox & Yeager, 2006, 2009Martinis et al, 2003;Xu et al, 2018). Near the primary peak, the Martian ionosphere is reasonably described by the idealistic Chapman theory under photochemical equilibrium (e.g., Mendillo et al, 2013Mendillo et al, , 2015Mendillo et al, , 2017.…”